1. Field of the Invention
The present invention relates to a printing medium conveying device and an image forming apparatus.
2. Description of the Related Art
In conventional image forming apparatuses, sheets such as documents and recording sheets are sequentially conveyed by a printing medium conveying device from a paper cassette to a sheet ejection unit. The printing medium conveying device includes a conveying path including a plurality of guiding members and a plurality of rollers that are driven to rotate, through which sheets are conveyed.
Once a printing operation is started, toner images formed in four colors (yellow, magenta, cyan, and black) image carriers are transferred to an intermediate transfer belt. Sheets that have been fed from the paper cassette one by one are conveyed through the conveying path to a secondary transfer device, where the toner images on the intermediate transfer belt are transferred to the sheet. The toner images are then fixed to the sheet in a fixing unit, and after that the sheet is ejected from the printing medium conveying device to outside the apparatus.
A skew correction device is provided in the conveying path for correcting the skew of the sheet occurred during the conveyance. The skew correction device abuts the leading edge of the sheet to the guiding member to align the position of the leading edge of the sheet to correct the position of the sheet so as to be parallel to a secondary transfer device. Specifically, by feeding the sheet excessively between a roller for the skew correction and a conveying roller located in the upstream thereof, the leading edge of the sheet is abutted to the guiding member. To align the sheet as described above, a space is required for forming slack on the sheet that has been fed due to evacuation of the sheet from the conveying path. For that purpose, sufficient distances and spaces need to be ensured between the rollers. In addition, if the used sheet is more rigid and thicker, larger distances between the rollers need to be ensured.
In recent years, however, the need has increased for a printer and other image forming apparatuses to cope with various types of sheet, print on a rigid thick sheet and print on a short-sized sheet such as a post card with a single printer or apparatus.
When slack is formed on a sheet between rollers using a skew correction device, for example, if the rollers are arranged with a short distance interposed therebetween, the conveying roller cannot feed a rigid thick sheet excessively due to the hardness of the sheet. To address such an issue, as disclosed in Japanese Patent Application Laid-open No. 2008-024507, for example, a mechanism is provided to separate a pair of rollers in the vertical direction and convey the sheet using other conveying rollers provided on the upstream thereof. With this structure, the sheet does not contact with the separated rollers, whereby slack is formed on the sheet along the long pitch between the upstream rollers and the rollers for the skew correction. The separation operation of rollers is performed, however, in a short time and typically for a few millimeters. The separation distance is too short to ensure a space to form sufficient slack on the sheet, causing an obstacle for feeding the sheet.
In view of the circumstances above, there is needed to provide a printing medium conveying device capable of ensuring a space for feeding sheets of different lengths to correct the direction of the sheets when printing media are conveyed.